This invention relates generally to a document reproduction system and, more particularly to an optical system in which a document on a document platen is moved along an object plane through an exposure zone and a reflected image is projected onto a moving flat photoreceptor at a rate faster than the rate of movement of the photoreceptor and in the direction opposite such photoreceptor movement.
Precession scan systems which expose images on a photoreceptor at a faster rate than the photoreceptor movement are known in the art. U.S. Pat. No. 3,454,335 (Caldwell) assigned to the same assignee as the present invention, discloses a method of projecting images on microfilm cards which are moved past a stationary lens and mirror system onto a drum photoreceptor. The drum and microfilm are moved at the same speed but the image is precessed, (i.e. moved in a direction opposite to the drum motion) onto the drum surface by means of a slit aperture which moves in a direction opposite that of the drum. This combined slit and drum rotation exposes the drum at a speed (rate) that is greater than the surface speed of the drum. This scanning system demonstrates two of the advantages gained by image precession: the process speed of the system can be set at a lower speed for equivalent copy rate (copies per minute) and the gap ordinarily present between images (due to the return time of the scan optics) can be reduced or eliminated if desired. Scan return velocities for a given copy rate can also be minimized for a given copy rate thus reducing scan accelerations, forces and vibrations. A problem with such a system, however, is that defocusing errors may result in developed images unacceptable for some systems. The errors are introduced because the image reflected from the drum mirror to the slit does not maintain perpendicularity to a tangential line at the drum surface.
This defocusing problem is addressed in U.S. Pat. No. 4,232,960 (Glab) assigned to the same assignee as the present invention. Glab solves the problem of field tilt in his particular scan system, by using linear and rotational motions of optical elements located near the drum surface to scan the image onto the drum at the angle corresponding to the angle of reflection of the image from the platen.
Another prior art device which utilizes precession onto a drum photoreceptor is disclosed in U.S. Pat. No. 3,650,621 (Lewis) also assigned to the same assignee as the present invention. Lewis discloses a system wherein an imaging device is moved in an arcuate path which is in a direction opposite to drum rotation to scan document on a curved platen onto the drum.
Precession onto a flat photoreceptor surface introduces different problems requiring a different set of solutions. U.S. Pat. No. 3,754,822 (Melrose) also assigned to the same assignee as the present invention, discloses a scan system wherein a platen and flat photoreceptor move synchronously in the same direction and the scan optics move in the opposite direction at a suitable speed. The disclosure and claims are limited to the specific system disclosed and do not address problems in systems which, for example, have a stationary object or which have a lens movement separate from that of a scan lamp movement. In higher volume copiers, a preferred method of scanning has been to utilize a full-rate, half-rate mirror system which maintains proper object-to-lens distance during the course of travel. Such a scan system is relatively heavy, however, and the above-mentioned flyback inertial problems constitute an important limitation on the speed of the surface.
Precession onto a flat photoreceptor surface is also disclosed in U.S. Pat. No. 4,336,995 assigned to the same assignee as the present invention. In this patent an object is imaged onto a photoreceptor by an illumination scanning system which is moving at a faster rate than the photoreceptor.
Another variation of precessing an image onto a flat photoreceptor is disclosed in U.S. Pat. No. 4,372,670 assigned to the same assignee as the present invention. In this patent a fixed document lying in an object plane is scanned by a modified dual rate mirror system where the second (slower) mirror is moving at less than half the speed of the first (scan) mirror thereby causing a resultant decrease in platen-to-lens distance. This decrease is compensated for, and image precession obtained, by moving a projection lens and associated mirror along the optical axis in a direction opposed to that of the photoreceptor.
A still further variation of a system for precessing an image onto a flat photoreceptor is disclosed in U.S. Pat. No. 4,351,605 also assigned to the same assignee as the present invention. In this patent, a reproduction device is operated in two modes, one of which enables precession when multiple copies are made of the same document. In the precession mode, scanning is accomplished by moving the document past a stationary exposure zone and imparting movement to the other optical components to provide the required precession rates.
None of the above-disclosed precession systems have utilized an imaging system wherein a document to be copied is placed on a platen which is movable in conjunction with scanning optical components. According to the invention, a document platen, exposure slit, projection lens and a belt photoreceptor are displaced at predetermined velocities and distances so as to form a precessed image of the document on the photoreceptor surface. An advantage of this type of semi-moving platen system is that the optical components can be packaged into a more compact physical space than that required of conventional stationary platen, lens, and scanning mirror configurations. More particularly, the invention is directed to an optical scanning system for projecting light images of a document lying on a platen in an object plane along an optical path onto a flat portion of a photoreceptor moving through an image plane, said system comprising,
means for moving said photoreceptor at a first rate of speed V.sub.4,
means for moving said platen in a direction opposite said photoreceptor motion and at a second speed V.sub.1 less than said photorecetor speed,
illumination means adapted to scan said document in a direction opposite said platen motion and at a third speed V.sub.2 greater than said photoreceptor speed, and
a scanning projection lens positioned along the optical path between said illumination means and said photoreceptor and moving at a fourth rate of speed V.sub.3 less than said photoreceptor speed and in the same direction as said photoreceptor whereby said scanned image is projected through said lens and imaged, in precession fashion, onto said photoreceptor in a direction opposite to the direction of travel of said photoreceptor.